To paraphrase The Bee Gees, Joseph Gordon-Levitt should be dancing. He's already done it in (500) Days of Summer, where he led an exuberant ensemble routine that out–Dr Peppered any Dr Pepper commercial. Then there was his smashing Saturday Night Live re-creation of Donald O'Connor's "Make 'Em Laugh" — like O'Connor, he has springs where his joints should be. If you can dance on a bike, he's pretty much done that too, in David Koepp's crackerjack bike-messenger thriller, Premium Rush. When we talk about dazzling physical actors, like Douglas Fairbanks or Marlon Brando, we might be thinking about people who use their bodies as brush strokes, as the beats between the notes, or as physical manifestations of submerged feelings, but there's another kind, too: Even when Gordon-Levitt isn't moving very much, he throws off the illusion of movement. Everything about him is alight; even his nerve endings seem to have a sense of humor.

There's no dancing in Gordon-Levitt's writing-directing debut, Don Jon, although the movie is so heavily reminiscent — in the good way — of Saturday Night Fever that an arm-swinging paint-can reverie wouldn't be out of place. But the picture is agile in every other sense.

It's a comedy that moves with a sense of purpose, as Gordon-Levitt does in the title role. His character is a Jersey lothario named Jon, who's earned the prefix "Don" because of his success with the ladies. Jon spends his days polishing his brawn at the gym — he looks almost unreal, like a Marine-turned–crocodile hunter as imagined by a video game designer. By night, he and his friends trawl the hot spots, looking for the most babe-o-licious girls.

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Jon's dirty little secret, though, is that while he thinks sex with real live women is OK, he prefers porn — not, it turns out, because the women he sees in porn are better-looking, or more controllable, or willing to do nastier stuff, but because he doesn't have to be anything for them. They free him in a way that the real women he meets don't.

Then he's bowled over by a bombshell at a local club, Scarlett Johansson's Barbara Sugarman, whom he thinks might be "the one." Barbara toys with him from the beginning, though in some ways her approach is sensible: She doesn't rush into bed, instead making him take her on a few dates beforehand. She loves romantic movies; Jon couldn't care less about them. (One of Don Jon's funniest bits involves the movie-within-a-movie Jon has to suffer through — it's called Special Someone, and it features Anne Hathaway and Channing Tatum cavorting and leaping like love-crazed morons.)

Barbara's a knockout, all right: Her nails are filed into pale, exquisitely varnished rectangles; her hair is styled in perfect dual spirals framing her fresh-from-Sephora face. But she's high-maintenance in all ways. "You'll be much happier if you tell me the truth," she tells Jon on their first date, but what he doesn't realize is that her idea of the truth is as restrictive as the squeeze of a boa constrictor.

At Barbara's behest, the somewhat aimless Jon begins taking night classes, where he meets a mysterious and obviously older woman — her name, as unglamorous as it gets, is Esther, and she's played by Julianne Moore. After catching Jon watching some downloaded smut on his iPod, Esther, amused rather than repulsed, presents him with a gift: a relic of '70s porn on DVD.

Esther explains the gift as if it were a dinosaur fibula, but she also wants Jon to know that his fixation is nothing new. And her entrance is also the point at which Gordon-Levitt's characters shift from being obvious, intentional cartoons into people with feelings. Even when his story starts getting serious, Gordon-Levitt always keeps it funny, and his cast is in on the joke.

Jon's Italian-American family is played by a killer triumvirate of Tony Danza, Glenne Headly and Brie Larson: They're stereotypes with beating hearts. Johansson is marvelous — her Jersey-girl diction is as precise as her character's guided-missile approach to courtship and marriage. And Moore, whose role at first seems thinly sketched, becomes the spirit of the movie: As Esther, she changes the story's course not so much with words or action but with a vibe. She underplays, as usual, and she's mighty like a rose.

Gordon-Levitt may be the movie's star, but he doesn't direct himself as its center. He uses his character as a kind of human flashlight, casting his beam on the people around him before looking inward. He plays Jon as egotistical, preening and, until the end, anything but sensual — he's ridiculous at first, a stylized man-panther who needs to dictate all the steps. Eventually, though, he gets the hang of what it means to be a dance partner.

Someday soon, someone needs to cast Gordon-Levitt in an honest-to-God musical. For now, Don Jon comes pretty close. It's not so much what Gordon-Levitt does; it's just something in the way he moves.